#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=1e5;
typedef long long LL;
inline LL read()
{
	LL x=0,f=1;char ch=getchar();
	while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}
	while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
	return x*f;
}
int main(void)
{
  cin.tie(0);std::ios::sync_with_stdio(false);
  LL t;cin>>t;
  while(t--){
    string a,b;cin>>a>>b;
    LL len1=a.size();LL len2=b.size();
    LL gcd=__gcd(len1,len2);
    LL lcm=len1*len2/gcd;
    string c="";string d="";
    for(LL i=1;i<=lcm/len1;i++){
        c+=a;
    }
    for(LL i=1;i<=lcm/len2;i++){
        d+=b;
    }
    if(c==d){
        cout<<c<<endl;
    }
    else cout<<"-1"<<endl;
  }
return 0;
}
